1. Field of the Invention
The present invention relates to a sheet post-processing apparatus for performing post-processing on a fold portion of a sheet bundle, a method of controlling the same, and a storage medium.
2. Description of the Related Art
In general, a post-processing apparatus is known which performs post-processing on a sheet (recording paper) having an image formed thereon. In this sheet post-processing apparatus, for example, sheets each having an image formed thereon are accumulated (stacked) and formed into a sheet bundle. Then, for example, the sheet post-processing apparatus staples a center in the sheet conveying direction or its vicinity of the sheet bundle and then delivers the sheet bundle in a booklet form after folding the same into two at the stapled portion. This post-processing is called saddle stitch bookbinding.
When performing saddle stitch bookbinding, for example, the sheet bundle subjected to stapling is pushed into a nip between a pair of fold rollers by thrusting the central portion of the sheet bundle using a thrusting member. Then, the sheet bundle is folded by the pair of fold rollers. Further, a folded portion of the sheet bundle is flattened by a flattening mechanism (see Japanese Patent Laid-Open Publication No. 2006-290588).
In the flattening mechanism of the sheet post-processing apparatus described in Japanese Patent Laid-Open Publication No. 2006-290588, a press roller is moved along a fold line of the folded portion while flattening the folded portion by the press roller, whereby the folded portion is flattened. Further, during flattening of the folded portion including the fold line, a clamper holds the sheet bundle so as to prevent the sheet bundle from returning toward the upstream side in the conveying direction.
The clamper for holding the sheet bundle holds the sheet bundle generally by pressure (biasing force) of a biasing member, such as a spring. Therefore, the clamper includes the spring, a clamping member, and a moving member, and the spring is disposed between the moving member and the clamping member.
In holding the sheet bundle, the moving member is moved toward the sheet bundle, whereby the clamping member is moved. And, after the clamping member reaches the sheet bundle, the moving member is further moved to compress the spring to create a spring force, whereby the created spring force causes the clamping member to hold the sheet bundle.
Incidentally, the thickness of the sheet bundle increases as the number of sheets of the sheet bundle is larger or as the basis weight of each sheet is larger. And, even if the amount of movement of the moving member is the same, the amount of compression of the spring increases as the thickness of the sheet bundle increases. If the compression of the spring increases, load applied to a drive source (e.g. motor) for driving the moving member increases. As a result, a braking distance becomes shorter over which the moving member is moved before it is stopped after the brake is applied to the motor.
However, if the braking distance becomes shorter, the moving member is stopped short of a target stopping position (target position). This sometimes makes the compression of the spring smaller than required, thereby making it impossible to obtain a sufficient holding force of the clamper. If it is impossible to obtain a sufficient holding force, i.e. if the holding force is insufficient, when the folded portion is flattened, the sheet bundle is pushed back toward the upstream side in the conveying direction, which makes it impossible to sufficiently flatten the folded portion.
If brake timing is delayed or moving speed of the clamp is accelerated so as to stop the clamping member at the target position for the purpose of holding a sheet bundle formed of a large number of sheets, in the case of a sheet bundle formed of a small number of sheets, the thickness of this sheet bundle is thin, and hence the load applied to the motor is small, making the braking distance longer, which sometimes causes the moving member to reach a limit position of movement thereof. If the limit position of movement of the moving member is reached, a driving section, such as the motor, is locked, causing a failure of the apparatus.